1. Field of the Invention
The present invention relates to an optical pickup for reproducing information recorded on a recording medium such as an optical disk, and in particular, to an optical pickup for reproducing information while removing crosstalk caused by adjacent information tracks, and an information reproducing apparatus having such an optical pickup.
2. Description of the Related Art
An optical disk has information tracks formed on its surface. Information is recorded on the information tracks. When reproducing information recorded on an information track, a light beam is applied to an information track, reflected light of the light beam is received, and information is extracted from the reflected light thus received. When a light beam is applied to an information track on which information to be reproduced has been recorded, the irradiation range of the light beam extends over not only the information track to which the light beam should be applied but also other information tracks adjacent to the information track to which the light beam should be applied. Because of demand of higher density of recorded information, this phenomenon is more remarkable in a recording medium having a small interval between information tracks. If the irradiation range of the light beam extends over other information tracks, crosstalk is caused thereby. The term "crosstalk" means that information on adjacent tracks gets mixed in information on the information track to be originally reproduced. If crosstalk occurs, it becomes difficult to accurately reproduce information to be reproduced.
As a method for removing crosstalk, the three beam method is known. In the three beam method, a light beam is applied to an information track from which information should be originally reproduced. In addition, a light beam is also applied to each of tracks adjacent to the information track from which information should be originally reproduced. In other words, a total of three light beams are applied toward an optical disk. From a light receiving signal based upon reflected light from an information track to be reproduced, light receiving signals based upon reflected light from two adjacent information tracks are subtracted to remove the crosstalk.
In a typical reproducing apparatus employing the three beam method, one light beam is divided into three light beams by using a diffraction grating. Among three beams resulting from the division, one is used as a main light beam whereas two remaining beams are used as subsidiary light beams for removing the crosstalk. The main light beam is applied to the information track having information to be reproduced, whereas the subsidiary light beams are applied to the adjacent tracks.
The subsidiary light beams are respectively applied to information tracks located on either side of the information track having information to be reproduced so as to be adjacent thereto. This means that irradiation positions of the two subsidiary light beams are disposed so as to be displaced respectively leftward and rightward from the irradiation position of the main light beam, assuming the extension direction of information tracks on which the irradiation position of the main light beam is located to be a center line. In other words, the subsidiary light beams are disposed so as to be displaced in the radial direction of the optical disk.
In addition, irradiation positions of the two subsidiary light beams are disposed so as to be equally displaced before and behind the irradiation position of the main light beam in the extension direction of information tracks. In other words, the irradiation positions of the subsidiary light beams are disposed so as to be displaced not only in the radial direction of the optical disk but also in the circumferential direction of the optical disk. If the three light beams are disposed so as to line up in a single file in the radial direction of the optical disk, then irradiation ranges of the light beams interfere with each other, and information on each information track cannot be detected accurately. In order to prevent this, the irradiation positions of the subsidiary light beams are disposed so as to displaced in the circumferential direction of the optical disk.
If the irradiation positions of the three light beams are disposed so as to be displaced in the circumferential direction of the optical disk, then light receiving signals obtained from respective light beams diverges in time. The temporal divergence of the light receiving signals can be removed by electrically delaying the light receiving signals.
It is now assumed that the spacing between the irradiation position of the main light beam and the irradiation position of each of the subsidiary light beams in the circumferential direction of the optical disk is L. The distance L can be represented as EQU L=F.times.(.lambda./D) (1)
where .lambda. is the oscillation wavelength of the light source, D is the pitch of the diffraction grating, and F is the focal length of an objective lens for focusing each light beam onto an information track corresponding thereto.
As for an optical material used for the diffraction grating, an optical material having the least possible coefficient of linear expansion .beta. is used from the viewpoint of stability against a change of the ambient temperature.
On the other hand, as the light source for emitting a light beam, a laser diode is used in many cases. The laser diode typically has poor temperature characteristics. As the ambient temperature changes, the oscillation length .lambda. of the laser diode changes. By the way, since an optical material having the least possible coefficient of linear expansion .beta. is used for the diffraction grating, a change of the pitch D of the diffraction grating caused by a change of the ambient temperature is small. As a result, a change of the oscillation frequency .lambda. of the laser diode caused by a change of the ambient temperature changes the spacing L (see the equation (1)). If the spacing L changes, time spacing between light receiving signals actually obtained from the reflected light of a light beam becomes different from time spacing preset in a removing circuit in order to electrically remove a time difference of an obtained light receiving signal. This results in a problem that the crosstalk cannot be removed accurately and the stability is poor provided that the ambient temperature changes.